Carbon dioxide production by individual mice as an index of behavioral and metabolic activity

A noninvasive method for simultaneous real-time determination of spontaneous locomotor activity (LA) and CO2 production (as minute volume expired CO2, or VECO2) in mice is described. As an apical measure of overall metabolic activity, VECO2 proved sensitive to changes in many physiological processes, including behavioral activity. In normal mice, LA (as photobeam breaks) and VECO2 were positively correlated (r = 0.73), and stable over repeated daily tests. Food deprivation for 18 hr overnight increased LA while decreasing VECO2. Stimulation of peripheral sympathetic activity, either by exposure to cold air or by injection of epinephrine, increased VECO2 without affecting LA. Intoxication with pentobarbital produced a biphasic change in both measures: anesthetic doses increased LA and VECO2 during induction and recovery, and reduced them during the period of anesthesia itself. These results characterize VECO2 as an easily quantifiable, composite index of behavioral and metabolic activity in mice. This measure, along with its co-variation with LA, may provide better information about toxic effects than any single screening test.